In Episode 4, check out UL's Fire Safety Research Institute (FSRI) Director, Steve Kerber, as he talks about how gases flow from high pressure to low pressure and how ventilation can impact the flow path:

When you’re on the fireground, it is important to understand the basic fire dynamics principle that gases flow from high pressure to low pressure. As a fire burns in a compartment, hot fire gases expand resulting in an increase in pressure. When a ventilation opening is made (doors, windows, roof) a low pressure exhaust is created, allowing gases to flow toward the low pressure outlet.

A flow path is the volume between an inlet and an outlet that allows the movement of heat and smoke from the higher pressure within the fire area toward the lower pressure areas accessible via doors and window openings. When you make ventilation changes to a structure, the flow of gases can either be used to for your tactical advantage or your detriment by making the fire environment worse.